For performing drive control of an AC motor, using a DC power supply, a drive method using an inverter is employed. In the inverter, an inverter drive circuit performs switching control. For example, the AC motor receives a voltage of which switching is performed according to the Pulse Width Modulation (PWM) control based on a comparison in voltage between a voltage command and a carrier.
The PWM control is generally performed by a so-called sinusoidal PWM control in which an amplitude of a phase voltage command to the AC motor is kept within a range of a voltage amplitude of the carrier. However, the sinusoidal PWM control cannot sufficiently cope with a high power. Therefore, e.g., Japanese Patent Laying-Open No. 2008-11682 (Patent Document 1) has disclosed an overmodulation PWM control using a phase voltage command of an amplitude larger than a voltage amplitude of a carrier.
Drive control of an AC motor disclosed in the patent document 1 switches the control among the sinusoidal PWM control, overmodulation PWM control, and rectangular wave voltage phase control. In the overmodulation PWM control disclosed therein, linear compensation of a voltage amplitude is performed on d- and q-axis voltage commands based on results of feedback of d- and q-axis currents. Further, it has disclosed that, in the sinusoidal PWM control and overmodulation PWM control, modulation can be selected from between the modulation in which a three-order harmonic is superimposed on a sinusoidal phase voltage command and the modulation in which the three-order harmonic is not superimposed. Particularly, it has been proposed to change switching conditions of the control mode according to presence/absence of the three-order harmonic.    Patent Document 1: Japanese Patent Laying-Open No. 2008-11682